Ionization pressure gauge



1950 M. YEATER 2,519,807

IONIZATION PRESSURE GAUGE Filed Oct. 10, 1944 INVENTOR. MAX L. YEATER ATTORNEY Patented Aug. 22, 1950 IONIZATION PRESSURE GAUGE Max L. Yeater, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application October 10, 1944, Serial No. 558,052.

The present invention pertains to the measurement of vacua and more particularly to ionization manometers for measuring gas pressure in thermionic vacuum and semi-vacuum tubes.

In the measurement of low pressure gases, the manometer most commonly employed for this purpose is the McLeod gauge whose operation depends on Boyles law and entails the use of mercury columns in capillary tubes for compressing a sample volume of gas under test. Another widely used manometer is the Pirani gauge whose operation is based upon the physical principles that the thermal conductivity of gas is a function of its pressure. The thermal conductivity is indirectly measured by ascertaining the change in resistance of a hot filament in a vacuum with pressure. As the pressure round the filament alters, so the speed at which heat is conducted away from the filament changes. This effect changes the filament temperature and hence its resistance. The resistance changes are recording by a Wheatstone bridge arrangement.

Another method by which gas pressure may be measured is by the use of ionization gauges wherein the electrons from a heated filament in a vacuum bulb are accelerated toward a collecting electrode while the positive ions formed during their passage are collected on a third electrode and the current recorded varies with the pressure prevailing in the bulb.

In recent years with the large scale production of vacuum and semi-vacuum thermionic tubes it has become increasingly desirable to measure gas pressure within a tube after the tube envelope has been sealed off. None of the measuring techniques above mentioned is capable of performing this operation since, in all cases, a sample of the gas must be introduced into the instrument. Moreover, since gas pressures in the order of one millimeter of mercury or less are encountered, or more specifically, in the range between 100 and 1000 microns, manometers hitherto available have not always been satisfactorily accurate at these pressures.

Accordingly it is the principal object of the present invention to provide an ionization gauge for measuring gas pressure in a vessel which has been sealed off.

It is another object of the present invention to provide an ionization gauge of the above type characterized by a higher degree of accuracy than has heretofore been available.

It is a further object of the invention to provide a pressure gauge which is simple both in construction and operation and which may be 5 Claims. (Cl. 175-183) readily calibrated. The technique employed in the present invention is based on the phenomenon that a low pressure gas will evidence a glow discharge between the electrodes of an electromagnetic wave oscillator providing the voltage and frequency are of the proper range. It has been noted that a minimum voltage at which the glow discharge first appears is a function of pressure for a given frequency, or, on the other hand, if the voltage is maintained constant, a glow discharge may be excited by raising the frequency.

The objects of this invention are fulfilled in a preferred embodiment wherein the sealed-off low pressure gas-filled tube is brought within the field of a radio frequency generator whose input voltage is adjustable. The voltage is increased to the point of glow discharge as indicated either visually or by means of a monitor circuit. The point of glow discharge being a function of gas pressure, by calibrating the voltage adjusting means, the pressure within the vessel may be directly indicated.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing. The scope of the invention will be pointed out in the accompanying claims.

In the accompanying drawing:

Figure 1 is a schematic circuit diagram of a preferred embodiment of the invention, and

Figure 2 is an alternative. electrode arrangement to be employed with the circuit of Figure 1.

Referring now to the drawing and more particularly to Figure 1 a preferred embodiment of the 1 invention is disclosed comprising a Hartley oscillator including a triode tube I0, a monitor circuit including a voltmeter H and a half-wave rectifier l2, and a testing circuit including a metal testing ring I3. The vacuum tube under test I4 is disposed within the ring l3, all of the elements therein being grounded through the connecting prongs.

The Hartley oscillator is of conventional design having a tank circuit formed by inductor l5 and variable capacitor [6, filament voltage for triode I0 is furnished by battery l'l while plate voltage is provided by battery l8. The power output of th oscillator is controlled by a potentiometer l9 shunted across battery ill, the sliding arm being connected through a radio frequency choke 20 to the plate of triode Ill. The output of the oscillator is inductively coupled to the monitor and test circuits by coils 2| and 22 respectively. One side of coil 2| is grounded, the other side being connected, through rectifier ii, to one terminal of a voltmeter II, the other terminal thereof being grounded. One side of coil 22 is grounded, the other side being connected to the testing loop I3.

In operation, condenser I6 is adjusted to a desired frequency. When potentiometer lLis varied as to increase the power output of the oscillator, a point will be reached where the radio frequency potential applied to tube I4 is sumcient to ionize the low pressure gaseous region therein. The ionization of tube I4 applies a relatively heavy load across the output of the oscillator which is indicated by voltmeter II which abruptly dips when the output of the oscillator is absorbed by the test circuit. The point of ionization will also, of course, be visually evident because of the glow of tube It. The minimum voltage at which the glow discharge first appears is a function of pressure for a given frequency. To calibrate the circuit in terms of gas pressure the readings may be compared with a standard Mc- Leod gauge.

Although in the method disclosed invFigure 1 the elements within the tube are used to provide an electrode whereby the radio frequency voltages are impressed upon the gaseous region of tu e I, this particular method is not essential to the operation of the ionization gauge. Referring to Figure 2 an alternative method is shown wherein the radio frequency voltages are applied to a thermionic tube 24 through a pair of external electrodes 23, the circuit in all other respects being identical with that illustrated in Figure 1.

It is to be understood that the voltage supplied to the oscillator may be maintained constant and the frequency varied to obtain pressure readings. However, since in most practical oscillators, varying the frequency will affect the power output thereof, unless the power variation is corrected for, the use of this method is inaccurate.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims, to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The-method of measuring gas pressure in a sealed-off vessel comprising the steps of generating high frequency wave energy, applying said energy to the vessel, maintaining the frequency substantially constant while adjusting the magnitude of said energy to the ionization point of the gas in said vessel and utilizing the 2,373,846

magnitude of applied energy at said ionization point to indicate said pressure.

2. Apparatus for measuring gas pressure in a sealed-oil vessel comprising an oscillatory wave generator, output electrodes for said generator disposed about the sealed-off vessel, means for adjusting the power output of said generator to the point of ionization of the gas contained within the vessel while the frequency of the output of said generator is maintained substantially constant, and means for utilizing the magnitude of said generator power required to attain said point of ionization to indicate said pressure.

, 3. Apparatus for measuring gas pressure in a sealed-01f vessel having metallic elements therein, said apparatus comprising an oscillatory wave generator, output electrodes for said generator, one of said output electrodes comprising a metallic member disposed externally of said vessel, the other of said output electrodes comprising the elements within said vessel, means for adjusting the power output of said generator to the point of ionization of the gas contained within said vessel while maintaining the frequency of the output of said generator substantially constant, and means for indicating the magnitude of said generator power required to attain said point of ionization as a measure of said pressure.

4. The method of measuring gas pressure in a closed envelope comprising the steps of applying a high frequency electromagnetic field of substantially constant frequency to the gas and measuring the voltage required to set up said field at ionization of said gas to indicate said pressure.

5. A device for measuring the gas pressure within an enclosed container comprising a high frequency oscillator capable of generating an electromagnetic wave of substantially constant frequency, means for varying the voltage of said oscillatory, means for applying said wave to the gas in said container, and means to indicate the magnitude Of said voltage substantially at the point of ionization of said gas as a measure of said pressure.

MAX L. YEATER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,607,823 Gustin Nov. 23, 1926 1,904,059 Kubach Apr. 18, 1933 1.917,465 Schweitzer July 11, 1933 2,266,114 Bartlett Dec. 16, 1941 2,370,475 Lemmers Feb. 27, 1945 Olken Apr. 17, 1945 

